1. Field of the Invention
This invention relates generally to a fuel cell system employing one or more positive temperature coefficient (PTC) ceramic heater and, more particularly, to fuel cell system employing PTC ceramic heaters in the end plates of a fuel cell stack and/or a hydrogen supply line for a compressed hydrogen storage tank.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electrochemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free protons and electrons. The protons pass through the electrolyte to the cathode. The protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA).
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. The fuel cell stack receives a cathode reactant gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen reactant gas that flows into the anode side of the stack. The stack also includes flow channels through which a cooling fluid flows.
The fuel cell stack includes a series of bipolar plates positioned between the several MEAs in the stack, where the bipolar plates and the MEAs are positioned between two end plates. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates that allow the anode reactant gas to flow to the respective MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode reactant gas to flow to the respective MEA. One end plate includes anode gas flow channels, and the other end plate includes cathode gas flow channels. The bipolar plates and end plates are made of a conductive material, such as stainless steel or a conductive composite. The end plates conduct the electricity generated by the fuel cells out of the stack. The bipolar plates also include flow channels through which a cooling fluid flows.
The membrane within a fuel cell needs to have a certain relative humidity so that the ionic resistance across the membrane is low enough to effectively conduct protons. This humidification may come from the stack water by-product or external humidification. The flow of the reactant gas through the flow channels has a drying effect on the membrane, most noticeably at an inlet of the flow channels. Also, the accumulation of water droplets within the flow channels from the membrane relative humidity and water by-product could prevent reactant gas from flowing therethrough, and cause the cell to fail, thus affecting the stack stability. The accumulation of water in the reactant gas flow channels is particularly troublesome at low stack output loads.
The end cells in a fuel cell stack typically have a lower performance than the other cells in the stack. Particularly, the end cells are exposed to ambient temperature, and thus have a temperature gradient that causes them to operate at a lower temperature as a result of convective heat losses. Because the end cells are typically cooler than the rest of the cells in the stack, water vapor more easily condenses into liquid water so that the end cells have a higher relative humidity, which causes water droplets to more readily form in the flow channels of the end cells. Further, at low stack load, the amount of reactant gas flow available to push the water out of the flow channels is significantly reduced. Also, at low stack loads the temperature of the cooling fluid is reduced, which reduces the temperature of the stack and typically increases the relative humidity of the reactant gas flow.
It is known in the art to heat the end cells with resistive heaters so as to compensate for convective heat losses. These known systems typically attempted to maintain the end cell temperature the same as the other cells in the stack by monitoring the temperature of the cooling fluid out of the stack. However, lower cell voltages for the end cells are still a problem even with the addition of such heaters.
Typically hydrogen is stored in a compressed gas tank under high pressure on the vehicle to provide the hydrogen necessary for the fuel cell system. The pressure in the compressed tank can be upwards of 700 bar. In one known design, the compressed tank includes an inner plastic liner that provides a gas tight seal for the hydrogen, and an outer carbon fiber composite layer that provides the structural integrity of the tank. At least one pressure regulator is typically provided that reduces the pressure of the hydrogen within the tank to a pressure suitable for the fuel cell system.
As the hydrogen is removed from the compressed gas tank, the pressure of the hydrogen in the tank decreases. When the pressure of a gas is reduced and the volume does not change, the temperature of the gas will also decrease. If the rate of the hydrogen flowing out of the tank is high enough and/or the temperature of the environment is low enough, the temperature in the tank can fall below 0° C. The low temperature of the hydrogen flowing from the tank may have adverse effects on the various components in the fuel cell system that receive the hydrogen, such as components in the fuel cell stack.